Apparatus for applying electrostatic charge to fibrous structure

ABSTRACT

An apparatus for electrostatically charging fibrous material being forwarded in a linear path that includes an ion gun and an opposed grounded target electrode positioned on opposite sides of the path. The electrode has a surface facing the ion gun that is covered by a dielectric material having a resistance measured at about 65* C. of between about 1 X 106 and 1010 ohms. Covering materials with resistances in this range minimize the disruptive effect on continuity of operation of polymer buildup on the target electrode.

United States Patent [72] Inventor Henry H. George [56] References CitedBerkeley gms, NJ. UNITED STATES PATENTS Q 25 33 3 3,277,526 10/1966Hollberg 18/8 1 y 3,387,326 6/1968 Hollberg et a1. 18/8 [45] PatentedMay 1971 3 456 156 7/1969 Kilb et al 18/8 [73] Assignee E. l. du Pont deNemours and Company y Wilmi gt D Primary Examiner-Dnald R. SchranAttorney-Howard P. West, Jr.

ABSTRACIT: An apparatus for electrostatically charging fibrous materialbeing forwarded in a linear path that includes an ion gun and an opposedgrounded target electrode posi- [54] gg i ggg y ggg igg tioned onopposite sides of the path. The electrode has a sur- 8 Claims 4 DrawinFi 8 face facing the ion gun that is covered by a dielectric material gg having a resistance measured at about 65C. of between about [52] US.Cl 18/8 1X10 and 10" ohms. Covering materials with resistances in [51]Int. Cl D01d 9/00 this range minimize the disruptive effect oncontinuity of [50] Field of Search 18/8; 317/4 operation of polymerbuildup on the target electrode.

/ l6 43 T0 SOLVENT RECOVERY ,7. POLYMER SOLUTION SUPPLY SE 20 24 60 DC 215 1 '9 u SOURCE 25 9 L r as as 7 O) v 40 2 j 27 l I l 3'] DC SOURCEBACKGROUND OF THE INVENTION This invention relates to an improvedapparatus for applying electrostatic charge to fibrous structures anddepositing them on a moving receiver to form a nonwoven sheet. Moreparticularly, it relates to an improvement in the target plate of acorona charging device which applies the electrostatic charge to thefibrous structures.

A means for charging a plexifilament is disclosed by Owens in US. Pat.No. 3,319,309, wherein horizontally flash-spun plexifilament immediatelycontacts an arcuate surface which spreads the plexifilament and directsit downward toward a moving collection belt. The arcuate surface may beoscillated to provide a traversing movement to the plexifilament. As theplexifilament continues downward, it crosses over an electricallyconductive grounded target plate. Spaced oppositely from the targetplate, and aimed at it, is an ion gun which provides a source ofionizing current. The ionizing current passes from the ion gun to thetarget plate through the gas-filled gap therebetween. Passage throughthe narrow gap between ion gun and target plate produces anelectrostatic charge on the plexifilament. The charged plexifilament isthen deposited on a moving receiver which is oppositely charged.

Associated with the flash spinning of fibrous structures is thegeneration of a small amount of tiny separated particles of solidifiedpolymer. While this is particularly true of the production ofplexifilaments, it is true to varying degrees for all flash spinning.These tiny particles become electrostatically charged like the endlessfibrous structure, but, owing to their small mass and momentum, becomeattracted to the target electrode, rather than to the collection belt,and gradually form an electrically insulating film on the electrode. Asthe thickness of the film increases, the potential drop across iteventually exceeds its dielectric strength. Tiny craters then formthrough the film, which craters become sources of back corona, i.e.,large current density plasma jets of both polarities. This situationdevelops rapidly, when it occurs, and can be detected as a precipitousincrease in current from the ion gun, i.e., corona discharge electrode.It causes neutralization of the charge on the plexifilament. Thereafter,the plexifilament does not spread out due to electrostatic repulsion ofits elements, is not attracted to and held upon the collection belt, andtends to float above and to collect nonuniformly on the collection belt.The target electrode at this point is said to be fouled, and thecondition is described as loss of electrostatic charging."

In order to postpone fouling, the target electrode has been constructedas an annulus and rotated so that the active area of its facecontinuously changes. An insulating film eventually forms over its wholeface, nonetheless. Further postponement of fouling has resulted when ascraper blade has been mounted near the top, inactive portion of therotating target plate to scrape off the deposited film. Still furtherpostponement of fouling is provided by maintaining a conductivesubstance on the target plate surface facing the ion gun. For example, aconductive liquid is applied continuously to the surface by wiping theliquid onto the surface as the plate rotates.

Each of the above-described methods for delaying target plate foulingand prolonging continuity of operation has operated satisfactorily inthe past and involves maintaining the target electrode at a lowresistance level. However, efforts to increase throughput of flash-spunfibrous structures have been accompanied by increased generation of thetiny separated particles of solidified polymer that foul the targetplates. In addition, higher throughputs have required higher currentdensities for efficient electrostatic charging of the plexifilaments. Asa result, target plates become fouled more rapidly than in the past.

SUMMARY OF THE INVENTION The purpose of the present invention is toprovide an improved apparatus for electrostatically charging acontinuous fibrous structure, which apparatus can operate efficientlyeven at high current densities and high throughput of the fibrousmaterial.

In accordance with this invention, there is provided an apparatus forpreparing nonwoven sheets from a continuous fibrous structure, includingmeans for flash spinning the fibrous structure, means for applying anelectrostatic charge to the fibrous structure by passage of thestructure through a charging zone between a corona discharge electrodeand a grounded target electrode and means for collecting the chargedfibrous structure on an oppositely charged or grounded moving receiver.The target electrode has a conductive base that has a surface facing thecorona discharge electrode which is covered by dielectric materialhaving a high electrical resistance measured at about 65 C.

There are several desired characteristics for the high resistancefacing. The facing must itself have a high dielectric strength in orderto prevent electric breakdown and back corona through the facing layer.The breakdown potential of the facing in volts is simply the dielectricstrength of the facing in volts per unit thickness multiplied by thefacing thickness. This potential should be several fold higher than theoperating voltage drop across the facing, which is given by the productof the current density j in 2, 2the volume resistivity of the facing ein ohm-cm and the facing thickness d in cm. The facing should besubstantially solid, uniform, homogeneous and reasonably resistant tochemical or physical degradation due to the environment or to theadverse effects of the corona discharge. The use of a rotating annularelectrode having a high resistivity facing is preferred because thefacing need not be exposed continuously to ion bombardment; only aboutone-third of the surface is in the field. In addition, the inactivesurface may be scraped continuously further minimizing buildup of thefouling layer. Equipment embodying the high resistance target plateshould operate with the target plate taking about 25 percent or more ofthe total applied voltage drop (e.g., between corona discharge electrodeand ground). The facing should have a resistance of between about l l0and 10 ohms. The preferred range is between about 2X10 and about 4 l0ohms. Below resistances of about 10 ohms, the effect of the facing inpostponing loss of electrostatic charging is lost. The upper values onresistance are determined from practical requirements such as the ratingon the available power supply and the insulation of surroundingsubstances. If the resistance is too high, too high a voltage will berequired to drive the current through the plate; hence current densitieswould have to be lowered and less efficient charging would occur. Sincehigh resistance facings on the target plate require higher voltages,care must be exercised to avoid arcing from the corona discharge needlesto an object other than the target plate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional schematicelevation of an apparatus embodiment useful in the practice of theinvention.

FIG. 2 is a partial cross-sectional schematic elevation of anotherapparatus embodiment particularly useful in the practice of theinvention.

showing the relationship of the target electrode to the ion gun and theconstruction of the electrode.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS FIG. I, shows aspinneret device 10, connected to a source of polymer dissolved in anorganic solvent. Polymer solution 12 under pressure is fed throughspinning orifice 14 into web forming chamber 16. The extrudate fromspinning orifice 14 is a plexifilament 7. Due to the pressure drop atspinning orifice 14 vaporization of solvent creates a vapor blast whichfollows the contours of baffle 18 concomitantly with plexifilament 7from spinning orifice 14 to collecting surface 9. Baffle i8 isoscillatably mounted and is powered through seal 11 to oscillate bymeans not shown. While oscillation of the baffle is not essential, itprovides suitable means for the preparation of wide sheets.

As shown, the target plate consists of two parts, a metallic portion 20and a high resistance surface portion 60. The target plate and ion gun22 are disposed on opposite sides of the path of advance of theplexifilament web 7 and downstream from the web-forming device. Themetallic portion 20 of the target plate is connected to ground by wire24 and microammeter 26 which indicates target plate current. Ion gun 22contains multiple needles 25, one of which is shown in FIG. 1. Eachneedle 25 of ion gun 22 is connected to a DC source 35 through resistor19. Each of the resistors is connected to the source of power throughconductor 21. Milliarnrneter 23 serves to measure ion gun current. A DCsource in the range of from 45 to 100 kilovolts may be used. The targetplate is so disposed that the vapor blast originating at spinningorifice 14 and the air flow pattern in chamber 16 carry plexifilamentweb 7 in close proximity to the high resistivity surface on the target.After passing through an ionized charging zone created by a coronadischarge between ion gun 22 and the target plate, the chargedplexifilament web 7 is deposited on collecting surface 9. The surfaceillustrated is a continuous belt forwarded by drive rolls 36. The beltis grounded or given an opposite charge to that imposed on web 7 bymeans of DC source 37 which is connected to the collecting apparatusthrough milliammeter 29 and lead 27. Due to the opposite polaritybetween web 7 and surface 9 the web in its arranged condition clings tothe surface as sheets 38 with sufiicient force to overcome thedisruptive influences of whatever vapor blast may reach this area.Surface 9 carries sheet 38 past compacting roll 44 and feeds the sheetout of chamber 16 through port 39 where it is collected on windup roll42. Flexible elements 40 across port 39 assist in the retention of vaporwithin chamber 16. Roller seals or labyrinth seals may also be used. Aconventional solvent recovery unit 43 may be beneficially employed toimprove economic operation.

An alternative apparatus embodiment useful in the practice of theinvention is shown schematically in FIGS. 2, 3 and 4. In FIGS. 2 and 4,the extrudate from orifice 14 of spinneret device is carried around thecurved surface of a lobed baffle 18 into close proximity with the highresistance surface 6%) of an annular target electrode. Baffle 18 iscontinuously rotated to impart oscillatory movement to the network offilm fibril material as it is deflected from the lobed surface. Annulartarget electrode 20' is coupled, for rotary movement about bafile 18' bymeans of ring 50 and pinion gear 52 attached to driven shaft 54. Themetallic portion 20' of the annular target electrode is connected tolead 24 through a contacting carbon brush 56. Ion gun 22 is U-shaped andis connected to a DC source through lead 21. FIG. 3 shows thearrangement of U- shaped ion gun 22' opposite the annular targetelectrode with the baffle 18' centered within the electrode. Needles 25are arranged in the lower tubular portion of the ion gun 22 such thatthe axes of the needles are generally perpendicular to the highresistance surface 60 of the target electrode (FIG. 4).

It is believed that postponement of the loss of electrostatic chargingby the improved apparatus of this invention can be explained as follows.The target plate, or passive electrode, has a facing of dielectric orhigh volume resistivity material. This facing in effect simulates aninfinite network of series and parallel resistors over the surface ofthe passive electrode. The simulated network acts as limiting resistorsand consequently any back corona breakdown which is initiated due to afouling deposit on the surface is restricted because of the limitedamount of current that can be drawn to that spot.

In preparing nonwoven sheets from continuous fibrous structures uniformcharging is required to maintain webs spread and to efficiently pin themto the collection surface. An electrostatic charge is imparted to thewebs by passing them through the unipolar region over the surface of thetarget plates. The unipolar region is established by the flow of ionsfrom the corona discharge device to the target plate. The chargecaptured by the fibrous structure is the same polarity as that of thedischarge electrodes and the coulomb repulsion forces to maintain theweb spread. The regions close to the discharge needles and the targetplate surfaces have the highest charge densities. The glow region nearthe discharge electrode, however, is bipolar (i.e., both negative andpositive charges are present) and tends to neutralize charges picked upby the web. Therefore, the unipolar region near the target plate shouldbe used although the web could be charged anywhere outside the coronaglow region.

When the tiny solid polymer particles, which are generated by theflash-spinning process enter the electrostatic field re gion, they getsprayed with ions, charged and then attracted predominantly to thegrounded target plate to form a nonconducting layer. The voltage dropacross the fouling deposit is given by the equation where V is thevoltage drop across the deposite, j is the current density inamperes/cmf, p, is the volume resistivity of the deposit in ohm-cm. andd, is the deposit thickness in cm.

When voltage drop V,exceeds the breakdown potential V of the foulingdeposite, breakdown of the deposit occurs in the form of craters andback corona takes place. Positive and negative ions are produced in thecraters. These ions in turn decrease the unipolarity of the region nearthe target plate and this results in loss of web charge.

When the target electrode has a facing material of high resistance inaccordance with the improved apparatus of this invention, back corona isdrastically reduced. The facing material functions to spread out thecharging field, thus reducing current density. When some breakdownoccurs in the fouling deposit and some back corona ensues, the highresistivity facing restricts the amount of current that can be drawn tothe point of breakdown. Substantial loss of web charge fromneutralization of oppositely charged ions in a back corona region isprevented for extended periods of time. Thus, the improved apparatus ofthis invention reduces the effects of fouling deposits, rather thanaltering the amount or nature of the deposit, thereby prolonging theperiods of efficient web charging. By comparison, at high throughput offlash-spun polyethylene plexifilaments, the best target plate of theprior art operated efficiently for between 4-8 hours, whereas theimproved target plates of this invention have operated satisfactorily inexcess of 4 days.

The series of tests and procedures that established the range ofelectrical resistances that are suitable for the target plates of theimproved apparatus of this invention is as follows.

The spinneret device of FIG. 2 is located over a moving belt similar tothe one shown in FIG. 1. Nonwoven sheets are prepared at high throughput(i.e., about 6080 pounds/hour/spinneret) from plexifilaments that areflashspun from solutions of linear polyethylene intrichlorofiuoromethane (Freon11).Target plates of the types shown inFIGS. 24 are used. The target electrode is conveniently 7.5 inches inouter diameter, 4 inches in inner diameter, and the metal portion aboutfive-sixteenth inch thick. The outer trailing edge of the target platecomprises an extension of the facing into which the rim of the metalportion sets. This edge extends one-half inch beyond the end of themetal portion and is contoured on the back side to avoid aerodynamicinterference with the descending plexifilament. A U-shaped coronadischarge electrode of the type shown in FIG. 3 is used. The dischargeelectrode has needles spaced three-fourths inch apart or needles spacedthree-eighths inch apart, located on the 3-inch-radius portion of theelectrode. The needle points are located five-eights inch from thetarget plate surface and about nine-sixteenths inch above the trailingedge of the target. A positive DC voltage is provided to the coronadischarge needles. A current density of at least 5 microamps/cm. and acharge on the plexifilaments in excess of 5 microcoulombs/gram andpreferably in excess of 8 microcoulombs/gram, are established asconditions of satisfactory charging.

The extent of fiber charging is indicated by the receiver or beltcurrent measured by microammeter 29 (FIG. 1). The belt mechanism iselectrically insulated from ground except for the path throughmicroammeter 29. It has been found that substantially all of the currentflowing from the corona discharge electrode is collected by either thetarget plate or by the collecting belt; thus where 1,, equals the iongun current, I equals target plate current to ground, and 1,, equalsbelt current to ground. The charge on the fibers is calculated from thebelt current 1,, and the polymer flow rate W by means of the equation:

wherein I, is the belt current in microamperes, W is the weight in gramsof fiber passing between the discharge electrode and target plate persecond, Q is the charge expressed in microcoulombs per gram.

The current density is determined from the target plate current toground 1 measured by microammeter 26 (FIG. 1) and A, the area of thecharging zone in cm. The area of the charging zone may be measuredeither in the spin enclosure or in the laboratory. In the spinenclosure, 16, the procedure is to stop the target plate from rotatingand allow the deposit from the spinning solution to build up on theplate. The deposits will outline the corona field. In the laboratory,polyethylene powder can be sprayed into the area between the dischargeelectrode and the target plate and it will take the outline of thefield. Atmospheric conditions in the laboratory must be the same as spinconditions. The area outlined by the deposit is the charging area. Forthe series of tests described herein, the area of the charging zonemeasured about 40 cm.

The resistance of the target plate facing is determined as follows.First, without any flow of plexifilaments in the corona charging zone,the voltage required to maintain a given corona discharge currentbetween the discharge electrode and a clean, bare metal target electrodeis measured from the DC power supply meters. The corona current ismeasured by microammeter 23. As noted above, the target electrode inthis series of tests is located five-eighths inch away from the coronadischarge needles. The total resistance of the /s-inchthick gap and theresistance of a metal electrode are then calculated from the current andvoltage by Ohms law (i.e., the voltage drop equals the product of thecurrent and resistance). The resistance of the gap alone isapproximately equal to the total resistance, or may be obtainedprecisely by subtracting the resistance of the metal target electrodefrom the total resistance. The resistance of the gap depends on the gascomposition, concentration, pressure and temperature as well as on thecorona current density. For example, a -inch-thick gap between thecorona-needle tips and the target plate surface, when filled with 92 to99 percent Freon 11 (the remainder being air) at 65 C., shows thefollowing relationship between current density and resistance:

- Gap resist- Corona. current density (n amps/0th.): ance (ohms) 3.75 1.78X 10 5.00 1. 50X 10 7.50 1. 21X 10 10.00 1. 05X 10 12.50 0. 92X 10 aThe measurement of corona discharge current and voltage required tomaintain this current is then repeated with a target electrode having ahigh resistance facing in place of the are metal target plate. Thedistance from the corona needles to the facing surface remainsfive-eighths inch and atmospheric conditions remain the same. From thesemeasurements, the total resistance of the system is determined by Ohm slaw as above. The resistance of the %-inch-thick Freon gap, asdetermined from the above table and the resistance of the metal portionof the target electrode are then subtracted from the total resistance,to give the resistance of the target plate facing. The resistance of themetal portion of the target electrode is always negligible compared tothe resistance of the gap or the resistance of the target plate facingsof this invention.

In the series of high throughput flash-spinning tests to determine thesuitable range of resistances for the target plates of this invention,several facing materials are tested. These tests include carbon-tilledpolymeric materials such as Hypalon chlorosulphonated-polyethylene,Teflon poly (tetrafluroethylene), and neoprene, glass, sprayedTeflonfilm, and bare metal. As an aid to maintaining good chargingefficiency with the bare metal target electrode, deposits on therotating electrode are removed by means of a scraper and a conductivefluid (i.e., Zelec UN mixed monoand dialkyl esters of phosphoric acid)is wicked onto the metal surface. Still another type of target electrodewith a high resistance surface is used. This plate comprises aperforated aluminum base having about 31.5 holes/in. each being0.090-inch diameter, an outer Ai-inch-wide epoxy rim to prevent coronaat a spray metal edge, and a sprayed 0.0022-inch-thick coat of paint onthe surface. In these tests, efforts are made to maintain for longperiods of efficient operation (i.e., corona current densities above5,u. amps/cm. and charge on the plexifilaments above 5 p colombs/gramand preferably in excess of 7p. coul./gm.). Test results withHypalonfaced target plates, the Hypalon being between one-sixth andoneeighth inch thick containing between 8.6 and 24 percent carbon, andhaving resistances between about 2 Xx 7 1.0 and 1.2 l0 ohms, shows theplexifilament charge of above about 10 p. coul./gm. are maintained forup to 96 hours. Each of the tests is terminated without loss ofefficient charging. A Az-inch-thick-glass faced target plate of 3.5 lOohm resistance efficiently charges plexifilaments at 8.8 pi couL/gm. formore than 3 hours without loss of efficient charging. The paint-coatedperforated target plate described above and having a resistance of about3 X10 ohms, also charges plexifilaments efficiently at 7.5 p. coul./gm.for more than 3 hours. Each of these tests was terminated for otherreasons.

By contrast to the satisfactory results described above, a target platewith a facing resistance of 4 10 ohms operates efficiently for less than10 minutes, at which time plexifilament charge reduces to about 2 a,.coul./gm. which is insufficient to permit collection of satisfactory,uniform, nonropy sheet. In another test, the target plate facingresistance is too high, 1.2 X10 ohms, causing the corona field to spreadout so for from the corona discharge needles that the current density inthe charging zone on the target plate is so low that not enough chargecan be placed on the plexifilament to make it pin to the belt. Byfurther comparison with the satisfactory results described above, thebare metal electrode that is scraped and has a conductive fluid wickedonto its surface, charges the plexifilament satisfactorily for less thanminutes; within that time the charge on the plexifilament drops from12.1 to less that 2 1/2 ,u cou1./gm., which is insufficient forsatisfactory collection of sheet.

From above results, it is concluded that satisfactory production ofnonwoven sheets can be obtained at high electrostatic charging rateswhen a target electrode is used which has a facing of between about 10and l" ohms and preferably between 2 X and 4 l0 ohms. -2

I claim:

1. In an apparatus for electrostatically charging a continuous fibrousmaterial being forwarded in a path from a source to a collecting meansincluding a charged ion gun and an opposed grounded target electrodepositioned on opposite sides of said path between said source and saidcollecting means, the improvement comprising: said target electrodehaving a surface facing said ion gun, said surface being covered with amaterial having a resistance measured at 65 C. of between 1 X10 and i0ohms.

2. The apparatus as defined in claim 1, said material having aresistance of from about 2 X10 to about 4 X10 ohms.

3. The apparatus as defined in claim 1, said collecting means beingoppositely charged with respect to said fibrous material.

4. An apparatus for preparing nonwoven sheets from a continuous fibrousstructure including means for flash spinning the structure. means forcollecting the fibrous structure and means for charging said fibrousstructure as it passes in a path from said spinning means to saidcollecting means. said charging means including a corona dischargeelectrode and an opposed grounded target electrode positioned onopposite sides of the path, and a direct current source connected tosaid discharge electrode, the improvement comprising: said target.electrode having a conductive base. said base having a surface facingsaid discharge electrode, said surface being covered with a materialhaving an electrical resistance measured at 65 C. of between 1 lO and10* ohms.

5. The apparatus as defined in claim 4, said material having anelectrical resistance measured at 65 C. of between about 2 X10 and 4 l0ohms.

6. The apparatus as defined in claim 4. said base being metal, saidmaterial being a carbon-filled polymeric material.

7. The apparatus as defined in claim 6, said polymeric material beingchlorosulphonated-polyethylene.

8. The apparatus as defined in claim 6, said polymeric material beingpoly (tetrafluoroethylene).

2. The apparatus as defined in claim 1, said material having aresistance of from about 2 X 107 to about 4 X 108 ohms.
 3. The apparatusas defined in claim 1, said collecting means being oppositely chargedwith respect to said fibrous material.
 4. An apparatus for preparingnonwoven sheets from a continuous fibrous structure including means forflash spinning the structure, means for collecting the fibrous structureand means for charging said fibrous structure as it passes in a pathfrom said spinning means to said collecting means, said charging meansincluding a corona discharge electrode and an opposed grounded targetelectrode positioned on opposite sides of the path, and a direct currentsource connected to said discharge electrode, the improvementcomprising: said target electrode having a conductive base, said basehaving a surface facing said discharge electrode, said surface beingcovered with a material having an electrical resistance measured at 65*C. of between 1 X 106 and 1010 ohms.
 5. The apparatus as defined inclaim 4, said material having an electrical resistance measured at 65*C. of between about 2 X 107 and 4 X 108 ohms.
 6. The apparatus asdefined in claim 4, said base being metal, said material being acarbon-filled polymeric material.
 7. The apparatus as defined in claim6, said polymeric material being chlorosulphonated-polyethylene.
 8. Theapparatus as defined in claim 6, said polymeric material being poly(tetrafluoroethylene).